GHSA-9j25-mm2h-2f76: LXD has arbitrary file write on host via `exec-output` symlink in crafted image

The record-output parameter of the /instances/$name/exec endpoint stores the output of the command in the exec-output directory of the instance. If exec-output is a symlink, file named exec_UUID.stdout and exec_UUID.stderr can be written to an arbitrary location where the .stdout file will contain arbitrary content. This behavior can be abused for arbitrary command execution.